Low Earth Orbit (LEO) constellations have certain advantages over higher orbit constellations due to their closer proximity to the earth. For example, using the same optical aperture a LEO satellite can provide higher resolution images of the earth than a satellite in a Medium Earth Orbit (MEO) or a Geostationary (GEO) orbit. Equivalently, a LEO satellite can provide a communications link to a ground terminal using lower transmit power and/or a smaller antenna due to its range advantage over a MEO or GEO satellite. This proximity to earth also results in lower launch costs as well.
A LEO constellation also suffers some disadvantages due to its proximity to the earth. Each LEO satellite is in view of a smaller portion of the earth at any given instant, so a LEO constellation requires a larger number of satellites.
In addition, non-geostationary satellite planes precess or regress with respect to the global map, due substantially to the earth's rotation. This relative movement creates the need for ground based communications equipment to track non-geostationary satellites. The geometry of this tracking requires ground equipment that generally possesses two or more unrestrained physical degrees of steering freedom.
What is needed is a satellite constellation which allows low cost ground stations that employ relatively simple satellite tracking capabilities. Further, there is a need for a constellation which provides global coverage and which allows the use of low cost ground stations in most parts of the world. In addition, there is a need to maintain continuous contact with at least one satellite in a satellite constellation without requiring ground stations to have multiple antennas.